Thermal imaging apparatus

Abstract

The radiometric measurement of the temperature of remote targets is usually done with a radiometer having a detector whose field of view is alternately switched between the target and a reference temperature source. The detector output is then used to change the reference source temperature to equal the target temperature. According to the invention the reference source temperature is not changed by the measuring apparatus. The difference in detector signals obtained from the target and reference source is used to calculate the target temperature from the known laws of thermal radiation and the known detectivity and spectral response of the detector. The invention can be incorporated in thermal imaging apparatus using the radiation detector scanned to form the thermal image as the radiometer detector. In such radiometric thermal imaging apparatus an array of detectors is scanned across the scene in a raster and a corresponding visual raster is produced. The output of one of the detectors is sampled at a point in the raster which is identified in the visual raster and which is aligned with the target. The output is also sampled when the detector is receiving radiation from the reference temperature source at a point between scans. The difference signal between these two samples is applied to a microprocessor, together with the reference source temperature, an estimate of target emissivity and the detector parameters. From these inputs the microprocessor is programmed to calculate the target temperature.

I claim: 
     
       1. An apparatus for measuring the temperature of a target, said apparatus comprising: a thermal radiation detector having a predetermined detectivity and spectral response, and capable of producing an output signal;   means for alternately forming a thermal image of the target on the detector and a thermal image of a reference temperature source on the detector;   means for sampling the detector output signal when the target image is formed on the detector to produce a target signal, and for sampling the detector output signal when the reference source image is on the detector to produce a measured reference signal;   means for forming a difference signal which is equal to the difference between the target signal and the measured reference signal;   means for calculating a calculated reference signal which is equal to a signal which would be obtained from the detector by forming a thermal image of the reference source on the detector after forming a thermal image of a body at a temperature of absolute zero on the detector;   means for producing a quotient signal by dividing the difference signal by an assumed emissivity of the target;   means for producing a calculated target signal by adding the quotient signal to the calculated reference signal; and   means for calculating the target temperature as that black body temperature which would produce the calculated target signal from the detector by forming a thermal image of the black body on the detector after forming a thermal image of a body at a temperature of absolute zero on the detector.   
     
     
       2. An apparatus as claimed in claim 1, characterized in that the sampling means comprises a first sample and hold circuit for the target signal, and a second sample and hold circuit for the measured reference signal. 
     
     
       3. An apparatus as claimed in claim 2, characterized in that: the apparatus further includes means for measuring the temperature of the reference source; and   the means for calculating the calculated reference signal and the means for calculating the target temperature both comprise a microprocessor, the relationship between the temperature of the reference source and the calculated reference signal and the relationship between the calculated target signal and the target temperature each being stored in the microprocessor as a polynomial expression, each polynomial expression having precalculated coefficients.   
     
     
       4. An apparatus as claimed in claim 1, characterized in that: the apparatus further includes means for measuring the temperature of the reference source; and   the means for calculating the calculated reference signal and the means for calculating the target temperature both comprise a microprocessor, the relationship between the temperature of the reference source and the calculated reference signal and the relationship between the calculated target signal and the target temperature each being stored in the microprocessor as a polynomial expression, each polynomial expression having precalculated coefficients.   
     
     
       5. A thermal imaging apparatus for forming a visible image of a thermal scene, and for measuring the temperature of a target in the scene, said apparatus comprising: a thermal radiation detector having a predetermined detectivity and spectral response, and capable of producing an output signal;   means for forming a thermal image of the thermal scene;   means for repetitively scanning the thermal image of the thermal scene across the detector;   a light source;   means for repetitively scanning an image of the light source across the field of view of an observer, said light source scanning being in synchronism with and corresponding to the scanning of the thermal scene across the detector; and   means for modulating the brightness of the light source with a signal derived from the detector output signal;   characterized in that the apparatus further comprises:   means for alternately forming a thermal image of the target on the detector and a thermal image of a reference temperature source on the detector;   means for sampling the detector output signal when the target image is formed on the detector to produce a target signal, and for sampling the detector output signal when the reference source image is on the detector to produce a measured reference signal;   means for forming a difference signal which is equal to the difference between the target signal and the measured reference signal;   means for calculating a calculated reference signal which is equal to a signal which would be obtained from the detector by forming a thermal image of the reference source on the detector after forming a thermal image of a body at a temperature of absolute zero on the detector;   means for producing a quotient signal by dividing the difference signal by an assumed emissivity of the target;   means for producing a calculated target signal by adding the quotient signal to the calculated reference signal; and   means for calculating the target temperature as that black body temperature which would produce the calculated target signal from the detector by forming a thermal image of the black body on the detector after forming a thermal image of a body at a temperature of absolute zero on the detector.   
     
     
       6. A thermal imaging apparatus as claimed in claim 5, characterized in that the sampling means comprises a first sample and hold circuit for the target signal, and a second sample and hold circuit for the measured reference signal. 
     
     
       7. A thermal imaging apparatus as claimed in claim 6 characterized in that: the apparatus further includes means for measuring the temperature of the reference source; and   the means for calculating the calculated reference signal and the means for calculating the target temperature both comprise a microprocessor, the relationship between the temperature of the reference source and the calculated reference signal and the relationship between the calculated target signal and the target temperature each being stored in the microprocessor as a polynomial expression, each polynomial expression having precalculated coefficients.   
     
     
       8. A thermal imaging apparatus as claimed in claim 6, characterized in that the apparatus further comprises means for increasing the brightness of the light source when its image is scanned onto the location of the visible image of the target in the field of view of the observer. 
     
     
       9. An thermal imaging apparatus as claimed in claim 8, characterized in that: the means for scanning the thermal image of the thermal scene and the means for scanning the image of the light source both comprise a polygonal mirror prism having an axis of rotation, said prism comprising a plurality of planar mirrors arranged at a variety of angles relative to the axis of rotation of the prism; and   the apparatus further comprises:   a linear array of thermal radiation detectors capable of producing output signals, one of which detectors produces the target signal and the measured reference signal;   a linear array of light sources;   means for modulating the brightness of each light source with a signal derived from a corresponding detector output signal;   means for generating a pulse when the prism is rotated into a position where the target image is formed on the detector which produces the target signal and the reference signal; and   means for applying the pulse to the first sample and hold circuit and for applying the pulse to the means for modulating the brightness of the light source which corresponds to the detector which produces the target signal and the reference signal.   
     
     
       10. A thermal imaging apparatus as claimed in claim 9, characterized in that: the apparatus further includes means for measuring the temperature of the reference source; and   the means for calculating the calculated reference signal and the means for calculating the target temperature both comprise a microprocessor, the relationship between the temperature of the reference source and the calculated reference signal and the relationship between the calculated target signal and the target temperature each being stored in the microprocessor as a polynomial expression, each polynomial expression having precalculated coefficients.   
     
     
       11. A thermal imaging apparatus as claimed in claim 8, characterized in that: the apparatus further includes means for measuring the temperature of the reference source; and   the means for calculating the calculated reference signal and the means for calculating the target temperature both comprise a microprocessor, the relationship between the temperature of the reference source and the calculated reference signal and the relationship between the calculated target signal and the target temperature each being stored in the microprocessor as a polynomial expression, each polynomial expression having precalculated coefficients.   
     
     
       12. A thermal imaging apparatus as claimed in claim 5, characterized in that the apparatus further comprises means for increasing the brightness of the light source when its image is scanned onto the location of the visible image of the target in the field of view of the observer. 
     
     
       13. A thermal imaging apparatus as claimed in claim 12, characterized in that: the means for scanning the thermal image of the thermal scene and the means for scanning the image of the light source both comprise a polygonal mirror prism having an axis of rotation, said prism comprising a plurality of planar mirrors arranged at a variety of angles relative to the axis of rotation of the prism; and   the apparatus further comprises:   a linear array of thermal radiation detectors capable of producing output signals, one of which detectors produces the target signal and the measured reference signal;   a linear array of light sources;   means for modulating the brightness of each light source with a signal derived from a corresponding detector output signal;   means for generating a pulse when the prism is rotated into a position where the target image is formed on the detector which produces the target signal and the reference signal; and   means for applying the pulse to the first sample and hold circuit and for applying the pulse to the means for modulating the brightness of the light source which corresponds to the detector which produces the target signal and the reference signal.   
     
     
       14. A thermal imaging apparatus as claimed in claim 5, characterized in that: the apparatus further includes means for measuring the temperature of the reference source; and   the means for calculating the calculated reference signal and the means for calculating the target temperature both comprise a microprocessor, the relationship between the temperature of the reference source and the calculated reference signal and the relationship between the calculated target signal and the target temperature each being stored in the microprocessor as a polynomial expression, each polynomial expression having precalculated coefficients.